* Don't reject Nscount > 0
IXFR request could have a SOA RR in the NS section
RFC 1995, section 3: https://tools.ietf.org/html/rfc1995
* Only one RR in the NS section is acceptable
* Remove URL from comment
Generalize the srv.Unsafe and make it pluggeable. Also add a default
accept function that allows to discard malformed DNS messages very early
on. Before we allocate and parse anything furher.
Also re-use the client's message when sending a reply.
Signed-off-by: Miek Gieben <miek@miek.nl>